Page 174 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004D-156 June 8, 2001 15:28

Cryogenic Process Engineering 17

field, and the critical current. These parameters can be prove as the temperature is reduced. The yield strength at
varied by using different materials or giving them special 20 K is considerably larger than at ambient temperature;
metallurgical treatments. For pure, unstrained metals, the Young’s modulus is 5 to 20% larger at the lower tem-
normal (atmospheric) transition temperature from the su- peratures, and fatigue properties, with the exception of
perconducting state to the normal state is very sharp. For 2024-T4 aluminum, are also improved at the lower tem-
alloys, intermetallic compounds, and ceramics, the transi- peratures. Since annealing of these metals and alloys can
tion temperature can be quite large. Superconductivity in affect both the ultimate and yield strengths, care must be
any of these materials, however, can be destroyed by sub- exercised under these conditions.
jecting the material either to an eternal or a self-induced The body-centered cubic (bcc) metals and alloys are
magnetic field that exceeds a predetermined threshold normally classified as undesirable for low-temperature
field. construction. This class includes iron, the martensitic
The alloy niobium–titanium (NbTi) and the intermetal- steels (low carbon and the 400 series of stainless steels),
lic compound of niobium and tin (Nb 3 Sn) are the most molybdenum, and niobium. If not brittle at room temper-
technologically advanced LTS materials presently avail- ature, these materials exhibit a ductile-to-brittle transition
able. Even though NbTi has a lower critical field and crit- at low temperatures. Cold working of some steels, in par-
ical current density, it is often selected because its metal- ticular, can induce the austenite-to-martensite transition.
lurgical properties favor convenient wire fabrication. The hexagonal close-packed (hcp) metals exhibit
There are several families of high-temperature super- mechanical properties intermediate between those of the
conductors under investigation for practical magnet ap- fcc and bcc metals. For example, zinc suffers a ductile-
plications. Most of these HTS materials are copper oxide to-brittle transition, whereas zirconium and pure titanium
ceramics with varying oxygen contents. Becauase of their do not. The latter and its alloys have an hcp structure,
ceramic nature, HTS materials are quite brittle. This has remain reasonably ductile at low temperatures, and have
introduced problems with some rather unique solutions been used for many applications where weight reduction
relative to the fabrication of flexible wires that can be and reduced heat leakage through the material have been
used in the windings of superconducting magnets. important. However, small impurities of oxygen, nitrogen,
Several of the low-temperature superconducting met- hydrogen, and carbon can have a detrimental effect on the
als, such as lead, brass, and some solders (particularly low-temperature ductility properties of titanium and its
lead–tin alloys), experience property changes when they alloys.
become superconducting. Such changes can include spe- Plastics increase in strength as the temperature is de-
cific heat, thermal conductivity, electrical resistance, mag- creased, but this is also accompanied by a rapid decrease
netic permeability, and thermoelectric resistance. Conse- in elongation in a tensile test and a decrease in impact
quently, the use of these superconducting metals in the resistance. Teflon and glass-reinforced plastics retain ap-
construction of equipment for low-temperature operation preciable impact resistance as the temperature is lowered.
must be evaluated carefully. The glass-reinforced plastics also have high strength-to-
weight and strength-to-thermal conductivity ratios. All
elastomers, on the other hand, become brittle at low tem-
D. Mechanical Properties
peratures. Nevertheless, many of these materials, includ-
A number of mechanical properties are of interest to the ing rubber. Mylar, and nylon, can be used for static seal
cryogenic engineer contemplating the design of a low- gaskets provided that they are highly compressed at room
temperature facility. These properties include ultimate and temperature before cooling.
yield strength, fatigue strength, impact strength, hardness, The strength of glass under constant loading also in-
ductility, and elastic moduli. creases with a decrease in temperature. Since failure oc-
curs at a lower stress when the glass surface contains sur-
face defects, the strength can be improved by tempering
1. Strength, Ductility, and Elastic Modulus
the surface.
It is most convenient to classify metals by their lattice sym-
metry for low-temperature mechanical properties consid-
erations. The fcc metals and their alloys are most often III. REFRIGERATION AND LIQUEFACTION
used in the construction of cryogenic equipment. Alu-
minum, copper, nickel, their alloys, and the austenitic Refrigeration in a thermodynamic process is accom-
stainless steels of the 18–8 type are fcc and do not exhibit plished when the process fluid absorbs heat at tempera-
an impact ductile-to-brittle transition at low temperatures. tures below that of the environment. Heat absorption at re-
Generally, the mechanical properties of these metals im- frigerating temperatures can take place in totally different